Snakebite Woes

Q: Have you noticed or had owners comment on the change in a horse's personality after a rattlesnake bite? My mare was bitten on the muzzle six years ago. We almost lost her, but after two days of treatment with various medications she survived. However, her personality seemed to change. She seems less people-friendly and less energetic. And, most notably, she went from an easy keeper to harder to keep weight on.

I wonder if it's from some sort of facial nerve damage from the bite? My vet said many horses that have been bitten by rattlesnakes show signs of congestive heart failure when necropsied years later. Have you ever heard of these long-term effects from a snake bite? What could be going on here?

Fiona, via e-mail

A: In the United States several hundred horses are bitten by poisonous snakes each year. Of these, it is estimated that 10-30% of snake bites result in fatalities. Rattlesnakes are responsible for most of the envenomations and for almost all snake-related deaths.

Rattlesnake venom can produce a local tissue reaction as well as systemic effects immediately--or shortly after--the bite occurs. The most consistent local effects of rattlesnake bites in horses are swelling and bleeding at the bite site, skin necrosis (tissue death), and local muscle necrosis.

Acute systemic signs of rattlesnake envenomation might include lysis of red blood cells (cell death by breaking of the cellular membrane, apparent in blood samples), reduction of the number of platelets in the bloodstream, blood clotting abnormalities, blood pressure disturbances, and muscle toxicity (including heart muscle damage, kidney toxicity, and liver damage).

Despite the fact the majority of the toxic effects of rattlesnake venom occur within hours to days of envenomation, some clinical signs might not be evident at the time of the bite. As your veterinarian mentioned, by far the best-described example of such long-term complications of rattlesnake envenomation in horses is heart failure. Clinical signs of heart failure might not become evident until several months after the bite. Other rare chronic complications of rattlesnake envenomation that have been described in humans and horses include wound complications at the bite site, respiratory disease, or respiratory compromise.